The short answer is that .308 and 7.62x51 are dimensionally the same. But .308 is a higher pressure cartridge, and the brass is slightly different between the two. If I remember, 7.62 is strong brass.

Short answer, you can often interchange the two, but check before hand. Putting .308 into a weapon designed for 7.62 could be dangerous because of .308's higher pressure. The weaker brass used in .308 also means your brass could get chewed up to the point you can't reload it. I know my roommate's CETME fires .308 just fine, but it deforms the brass on extraction.

It's generally pretty safe to put 7.62 NATO in a weapon designed for a .308 without any problems. Note that if you're using a semi-auto .308 and put 7.62 NATO in there, you might see some problems on cycling because 7.62 NATO doesn't generate as high a pressure to cycle the action.

If you signed up for Aims mailinglist they send out specials once in a great while. I jumped on the 2A from AIM when they sent me an email saying free 140 rnd battle pack and free shipping. once the gun came in it looked pretty good. Not as nice as my SKS but for what it went through it is a nice gun.

I know my roommate's CETME fires .308 just fine, but it deforms the brass on extraction.

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The brass deformation is caused by the fluted chamber. CETMEs, H&K rifles and most of the clones have flutes machined into their chamber to aid in extraction and increase reliability in extreme operating environments. A lot of case necks get dented on the ejection port too. They make a rubber clip on buffer that solves that problem. They'll shoot .308 till the cows come home without worries. Their built like tanks.

I've have had/seen issues shooting bullets over 160 grains in weight in an HK91, and 911. If you try to eject a chambered round of 168 or 175 match ammo the bullet will remain stuck in the chambers throat and all you'll eject is the cartridge case and powder.

There's also a reliability issue with the cam locking shoulder thing in the bolt if you use heavy bullets a whole lot. The cam profile is designed to function optimally with 147 grain nato spec ammo. The heavier bullets firing cycle timing/chamber pressure is different and can cause problems like breaking the locking rollers off. All the gory details can be found in a back issue of Small Arms Review.

You can shoot heavy rounds in the rifles but to do it right you need to get a cam locking shoulder with the correct profile. These can be found for the HK's I'm not sure about the CETME's and PTR's.